The present invention is generally directed to digital signal processing. More specifically, the present invention is directed to minimization of system latency in signal processing paths including digital control loops.
The use of digital signal processing for communication systems, such as cable and satellite transmission systems, has long been known in the art. Presently, these digital communications are in widespread use in establishing links between nearly all types of communication devices in which two or more such devices are in need of high quality communication with one another. As a result, these systems allow for the utilization of sophisticated communication applications in which each member can communicate with other members and other devices. Such digital signal processing devices have been developed in a the intended use. One form of digital signal processing device in use today in communication systems is an active noise cancellation (ANC) device. The ANC-device is most often used in a sound environment where there are one or more disturbance or noise signals that tend to obscure the desired or target signal. The conventional ANC device generally includes a feedback circuit which uses an input transducer such as a microphone to detect ambient noise and an output transducer such as a loudspeaker or receiver to both generate an antinoise signal to cancel the ambient noise and to deliver the desired signal. The particular circuit elements vary from implementation to implementation.
Currently, ANC is achieved in analog form by introducing a canceling antinoise signal. The actual noise is detected through one or more microphones. An antinoise signal of equal amplitude and opposite phase is generated and combined with the actual noise. If done properly, this should result in cancellation of both noises. The amount of noise cancellation depends upon the accuracy of the amplitude and phase of the generated antinoise signal. ANC can be an effective method of attenuating low-frequency noise which can prove to be very difficult and expensive to control using passive noise control techniques.
Turning first to FIG. 1, a block diagram of a first prior art feedback active noise cancellation system 10 as disclosed in U.S. Pat. No. 4,455,675 and 4,644,581 is shown. The system 10 has as input a desired signal and a Noise signal and generates an output signal. For discussion purposes, it will be assumed that the desired signal is an input voice (Vin) signal and that the output signal is an output voice (Vout) signal. The Noise signal is considered to be any disturbance signal in the sound environment other than the desired signal. The Vout signal is a combination of the Vin signal, the Noise signal, and an antinoise signal generated by the system 10. As noted above, in theory the antinoise signal exactly cancels the Noise signal leaving only the Vin signal without attenuation as the Vout signal. In fact, this is not always the result. The system 10 attempts to achieve as high a gain as possible in the overall loop within a predetermined frequency range while maintaining the system stability. The forward path of the system 10 includes a compressor 12, a compensator 14, a power amplifier 16, and a receiver 18. For example, the receiver 18 could be any output transducer including a loudspeaker. The feedback path of the system 10 includes a microphone 20 as an input transducer and a microphone preamplifier 22. The Vin signal and the feedback path signal are combined in a first summation node 24. The forward path signal and the Noise signal are combined in a second summation node 26.
Turning now to FIG. 2, a block diagram of a second prior art feedback active noise cancellation system 30 as disclosed in U.S. Pat. No. 5,182,774 is shown. One will note that the system 30 has similarities with the system 10 of FIG. 1 except that the forward path includes a high-pass filter 32, a low-pass filter 34, and a mid-range filter 36 in combination with the receiver 18. Further, the feedback path adds a high-pass filter 38 to the microphone 20 and the microphone preamplifier 22.
Turning now to FIG. 3, a block diagram of a third prior art feedback active noise cancellation system 40 as disclosed in U.S. Pat. No. 5,604,813 is shown. In this case, a boost circuit 42 has been added outside of the closed loop, that is, before the first summation node 24, to equalize the desired signal. The feedback path of the system 40 includes the microphone 20, a plurality of band-pass filters 44, and a low-pass filter 46.
While widely used in the art, the conventional analog approach for reducing noise in a system is not without its problems. ANC systems are theoretically able to null the noise by generating a phase-inverted antinoise signal, however, as a practical concern, the various components of the system such as the input and output transducers will introduce certain undesirable delays. These delays may adversely affect the frequency range over which noise can be cancelled, the degree to which noise can be cancelled, and the stability of the noise-cancellation system. It is therefore desirable to be able to minimize the associated delays in the circuit. Likewise, it is also desirable to be able to adjust the circuit to compensate for component variation and manufacturing tolerances and for usage conditions to maximize the noise-cancellation frequency range and noise-cancellation ratio. Such adjustability is difficult to achieve using analog techniques. Another desirable function that can prove difficult in the analog domain is the equalization of the signal for frequency-dependent attenuation caused by subsequent processing functions.
A method and an apparatus for minimizing latency in digital signal processing paths is disclosed. One example is an active noise cancellation device. The system includes a digital closed feedback loop having a forward path and a feedback path. The forward path includes a compensation filter, a digital-to-analog converter, and an output transducer. The feedback path includes an input transducer, a feedback delta-sigma modulator, and a feedback sampling-rate converter. An input signal is processed in one of several ways into a processed digital input signal having a preselected intermediate sampling rate. Through the feedback path, an analog output signal is processed into a digital feedback signal having substantially the same preselected intermediate sampling rate. The processed digital input signal and the digital feedback signal are combined and processed through the forward path to produce an anti disturbance signal that is combined with a disturbance signal to form the analog output signal.